CONNECTION BETWEEN EMBRYO AND MOTHER. 667 



The connection between embryo 



The lowest Mammals, the Duckmole 

 and the Porcupine Ant-Eater {Echidna), 

 resemble Birds and most Reptiles in 

 bringing forth their young as eggs, i.e. 

 in being oviparous. The eggs are large, 

 with a considerable quantity of yolk, 

 and after fertilisation divide partially, 

 i.e. exhibit meroblastic segmentation 

 like the eggs of Birds and Reptiles. 

 The tunic formed round about them 

 in the Graafian follicles of the ovary 

 consists, as in Birds and Reptiles, of 

 a single layer of cells. Development 

 begins in the oviducts, but the eggs 

 are in no way attached to the wall. 

 They are laid in a nest by the Duck- 

 mole ; in the Echidna they are hatched 

 in a slight, periodically developed, ex- 

 ternal pouch. 



{b) In the Marsupials the connec- 

 tion between mother and offspring has 

 become closer. The embryo is born 

 alive, but prematurely and after a short 

 gestation. Till recently it was believed 

 that during its intra-uterine life it was 

 either not attached to the wall of the 

 uterus at all, or only to a slight extent 

 by a yolk-sac placenta. It is now 

 known, however, that, in Perameles at 

 least, there is not only an efficient 

 yolk-sac placenta, but a distinct, though 

 small, allantoic placenta. The general 

 absence of a placenta in Marsupials, 

 and the small size of the allantois, 

 must therefore be ascribed to degener- 

 ation, and not to a primitive condition. 

 The presence of a yolk-sac placenta in 

 Marsupials is not in itself of great 

 importance, for a connection between 



and mother. — (a) 



( Ornithorhynchus) 



Fig. 29 r. — Development 

 of hedgehog. Three 

 early stages. — After 

 Hubrecht. 



I. Shows internal vesicle of 

 hypoblast ; the disc and 

 external sheath of epi- 

 blast. II. Shows villi 

 arising from trophoblast ; 

 the disc of formative epi- 

 blast (Ep.) ; the blasto- 

 dermic vesicle (B.v.). 

 III. A more advanced 

 stage ; Tr. , trophoblast ; 

 E6., disc of formative epi- 

 bfast ; Bv. ' 



vesicle : H. 



blastodermic 

 hypoblast. 



the yolk-sac of the 



embryo and the wall of the oviduct exists in two Elasmo- 



